Nusselt Number

Definition

The Nusselt number is a dimensionless quantity used in heat transfer calculations, representing the ratio of convective to conductive heat transfer across a boundary. It helps quantify how effectively heat is being transferred by convection compared to conduction, and is essential for analyzing heat transfer in fluids and designing heat exchangers.

5 Must Know Facts For Your Next Test

1. The Nusselt number can be expressed as Nu = hL/k, where h is the convective heat transfer coefficient, L is a characteristic length, and k is the thermal conductivity of the fluid.
2. In laminar flow, the Nusselt number is often constant, while in turbulent flow, it varies with Reynolds number and Prandtl number.
3. Common Nusselt number correlations include empirical relationships derived from experimental data for specific geometries and flow conditions.
4. A higher Nusselt number indicates more efficient convective heat transfer relative to conduction, making it a key factor in optimizing heat exchanger designs.
5. The Nusselt number plays a crucial role in the thermal performance assessment of systems involving heat exchangers, cooling towers, and other engineering applications.

Review Questions

• How does the Nusselt number relate to the efficiency of heat transfer in different flow regimes?
  • The Nusselt number indicates the efficiency of heat transfer by comparing convective and conductive heat transfer mechanisms. In laminar flow, it tends to be lower and constant, suggesting less efficient heat transfer. Conversely, in turbulent flow, a higher and variable Nusselt number signifies enhanced convective heat transfer due to increased mixing. This understanding helps engineers optimize designs for better thermal performance across various applications.
• Discuss how the Reynolds number and Prandtl number influence the Nusselt number in practical applications.
  • The Reynolds number and Prandtl number are key factors that influence the Nusselt number, especially in turbulent flow scenarios. As the Reynolds number increases, indicating a shift from laminar to turbulent flow, the Nusselt number typically increases due to enhanced mixing and heat transfer. Similarly, the Prandtl number affects thermal and momentum diffusivity; higher values often lead to higher Nusselt numbers. This relationship is essential for engineers when predicting heat transfer rates in various systems.
• Evaluate how the Nusselt number impacts design considerations for heat exchangers in chemical engineering processes.
  • The Nusselt number directly impacts the design and efficiency of heat exchangers by informing decisions about flow arrangements, surface area requirements, and material selection. Higher Nusselt numbers indicate better convective heat transfer performance, which allows for smaller and more efficient designs. Engineers utilize correlations and empirical data relating Nusselt numbers to specific geometries and operating conditions to optimize thermal performance while balancing cost and material considerations in chemical engineering applications.

Related terms

Reynolds Number: A dimensionless number that characterizes the flow regime in a fluid, defined as the ratio of inertial forces to viscous forces.

Prandtl Number: A dimensionless number that relates the rate of momentum diffusion to the rate of thermal diffusion in a fluid.

Heat Transfer Coefficient: A measure of the heat transfer capability of a material or fluid, it quantifies the amount of heat transferred per unit area per degree of temperature difference.